Pneumatic radial tyre

Abstract: A pneumatic radial tyre, in which tyre weight is reduced, maintaining steering stability and high speed running performance. The tyre comprises a non-metallic cord breaker belt (7) disposed radially outside a carcass (6), and a jointless band belt (8) disposed radially outside the breaker belt (7). The band belt (8) comprising of at least one hybrid cord (23) wound spirally and continuously in the circumferential direction of the tyre at an angle of 0 to 3 degrees with respect to the tyre equator. The hybrid cord (23) comprises a low elastic modulus thread (22) and a high elastic modulus thread (21) which are final-twisted together. The low modulus thread (22) comprises at least one low modulus fibre first-twisted and having an elastic modulus of not more than 2000 kgf/sq.mm. The high modulus thread (21) comprises at least one high modulus fibre first-twisted and having an elastic modulus of not less than 3000 kgf/sq.mm. In the band belt ply, the count (E) of the hybrid cord per 5cm width, the stress (F1) in kgf of the hybrid cord at 2% elongation, and the stress (F2) in kgf of the hybrid cord at 6% elongation satisfying the following relationships: FI x E < 60; and F2 x E > 150.

Description: The present invention relates to a pneumatic radial tyre, in which a reduction of the tyre weight can be achieved, while maintaining its steering stability and the high speed running performance by preventing the tread portion from being cut and tread groove bottom from being cracked which are caused during running. Recently, from a point of view of preservation of the earth's environment, energy-saving and the like, there is a great demand for cars whose fuel consumption is less and in the tyres, it is required to reduce the tyre weight. Hitherto, in order to reduce the weight of a radial tyre, the belt has been widely made from high elastic modulus organic fibre cords instead of steel cords. Aromatic polyamide fibre cords (hereinafter aramid cords) are typical thereof. Also, in a radial tyre for high speed and heavy load use, in order to prevent the belt from being lifted during running, a so called jointless band or bandage belt (hereinafter JLB belt) is provided outside the breaker belt. The JLB belt is formed by winding a long narrow strip spirally and continuously in the tyre circumferential direction around the belt. In a tyre having the above-mentioned construction, as the inner pressure increases, the bottoms of the tread grooves are subjected to a large stress, and the groove bottoms are liable to be cracked during running. Therefore, tyre durability tends to be decreased. When the tyre is demoulded after vulcanisation, the belt layer contracts radially to decrease its diameter, and the radius of curvature of the tread portion is greatly altered from the radius of curvature of the mould. If priority is given to the hoop effect for preventing lifting during running, it becomes hard to press the raw cover tyre into the mould impression or pattern during vulcanisation, and as a result, the moulding becomes inferior. As a method for solving this problem, a JLB structure is disclosed in Japanese Patent application laid open No. 1-247204, in which a hybrid cord made of a high elastic modulus fibre strand (hereinafter high elastic modulus thread) and a low elastic modulus fibre strand (hereinafter low elastic modulus thread) is used. In this method, if non-metallic cords are used in the belt to reduce the weight, the problem of tyre contraction during vulcanisation also arises. That is, as the belt has less pull-resistance, when the tyre inner pressure is increased during vulcanisation, the tyre inflates and the band is elongated. As a result, moulding is superior. When the inner pressure is decreased after the vulcanisation, the cords return to the almost same length as the original length while leaving a small permanent elongation (set). As the rubber accompanies or follows such elongation and contraction, the diameter of the tyre is increased when the tyre is inflated with air. This means that the tread rubber is in an elongated state, in which the cut resistance of the tread rubber and crack resistance of the tread groove are decreased. In Japanese patent application laid open No. 3-279004, an invention was disclosed in which a hybrid cord is used to decrease the shear between belt plies, maintaining the fitness for and the case of fit into to the vulcanising mould. Also in this application, for example Fig.2 shows that the band is not limited to only a JLB, and the stress at 6% elongation is 1 to 1.5 kgf. Therefore, if the cord count is set at a usual level of about 50, the product of the stress and cord count is about 50 to 70 kgf. In Japanese patent application laid open No. 4-169304, an invention was disclosed in which the quantity of the steel cords in a steel cord belt layer is properly reduced by using a JLB made of hybrid cords However, there is no disclosure about the above-mentioned cut resistance and crack resistance. In EP-A-0 454 432 which forms the preamble of claim 1 and in Japanese patent application laid open Nos. 4-8605 and 4-356205, a band made of a ribbon of rubberised parallel hybrid cords (hereinafter hybrid band) is disclosed. While there is no disclosure about the twist number for the hybrid cord, the stress at 3% elongation is described as being not more than 2.5 g/d. Further, it is explained that when less than 1g/d, it is difficult to manufacture such a cord. In British patent 2,064,445, the stress-elongation curves of various cords are shown in Fig.3, wherein a 3 X 7 metallic cord displays a low stress up to about 6% elongation, but when the elongation exceeds this value, the stress increases abruptly. In the disclosed invention, such a cord is used in a belt edge band, but two or more breaker belts made of metallic cords are provided. Therefore, it is not a structure intended to reduce weight. In a pneumatic radial tyre, when non-metallic cords are used in its belt instead of steel cords to achieve a weight reduction while maintaining high speed durability and steering stability, the belt is required to be improved in hoop effect to prevent lifting during high speed running, while displaying a proper elongation during vulcanisation which allows easy inflation of the tyre to fit the mould and form the pattern. Even if both the requirements are satisfied, when the elongation during vulcanisation and hoop effect are such that the change in the tyre diameter due to the margin of a tyre contraction is large, there is a problem that the tread portion is inferior in cut resistance and crack resistance. It is therefore, an object of the present invention to improve the total quality of a tyre by solving the above-mentioned problems while satisfying the recent requirements for the high speed running performance and weight reduction. According to the present invention, a pneumatic radial tyre comprises a toroidal carcass ply, a breaker belt disposed radially outside the carcass ply, and a band belt disposed radially outside the breaker belt, characterised by the breaker belt comprising at least one ply of parallel non-metallic cords, the band belt comprising a ply of at least one hybrid cord wound spirally and continuously in the circumferential direction of the tyre at an angle of 0 to 3 degrees with respect to the tyre equator, the hybrid cord comprising a low elastic modulus thread and a high elastic modulus thread which are final-twisted together, the low elastic modulus thread comprising at least one low elastic modulus fibre first-twisted and having an elastic modulus of not more than 2000 kgf/sq.mm, the high elastic modulus thread comprising at least one high elastic modulus fibre first-twisted and having an elastic modulus of not less than 3000 kgf/sq.mm, in the band belt ply, the count (E) of the hybrid cord per 5cm width, the stress (F1) in kgf of the hybrid cord at 2% elongation, and the stress (F2) in kgf of the hybrid cord at 6% elongation satisfying the following relationships: FI x E < 60; and F2 x E > 150. For the low elastic modulus fibre, nylon fibre, polyester fibre or the like can be used. On the other hand, for the high elastic modulus fibre, aromatic polyamide fibre, polyvinyl alcohol fibre, carbon fibre, glass fibre or the like can be used. However, the combination of the low modulus nylon fibre and high modulus aromatic polyamide fibre is preferably used in view of the balance of the stress between the elongation range of under 6% and that of over 6%. In the present invention, by improving and specifically defining the cord construction and twist, it becomes possible to manufacture a cord having a stress of less than 1 g/denier, whereas the above-mentioned Japanese patent application laid open Nos. 4-8605 and 4-356205 explained such a hybrid cord is difficult to manufacture. Therefore, the drawback of the conventional hybrid cord can be eliminated and the present invention has been accomplished. Accordingly, the tyre weight is reduced because a non-metallic belt is used. Further, the hoop effect is improved and the lifting during high speed running is prevented because a JLB belt is used. Furthermore, the band has no joint, which generates less vibrations, and the band has an excellent durability. Usually a non-metallic belt is used, the belt rigidity tends to become insufficient. However, such a loss of belt rigidity can be compensated by the use of a hybrid band made of a high elastic modulus thread and a low elastic modulus thread twisted together. Further, the rigidity can be increased by decreasing the final-twist by decreasing the first-twist for the high elastic modulus thread and also by providing an extensibility for the hybrid cord through an adhesion process and heating processes therefor. In the hybrid cord, by setting the thickness of the low elastic modulus thread at not more than 1/2 of the thickness of the high elastic modulus thread, and by setting the first-twist of the low elastic modulus thread at not more than 1/2 of the final-twist number, the product of the stress (kgf) of the hybrid cord at 2% elongation multiplied by the cord count is set at less than 60. Therefore, the cord can be easily elongated by a small load corresponding to the inner pressure at the time of vulcanisation. Further, the elastic modulus required under heavy load can be maintained by setting the product of the stress (kgf) at 6% elongation (corresponding to the inner pressure in actual service conditions) multiplied by the cord count at more than 150. Given that cords are arranged in a certain direction in parallel with each other, the above-mentioned cord count is the number of the cords counted in the arranging direction in a unit width (5cm width in this invention). Since the cord having the special elongation characteristic is used in the hybrid band, the band can be expanded during vulcanisation, where the expansion corresponds to the diameter difference between the raw tyre and mould. After the vulcanisation, the elongation stress of the band decreases the tyre diameter. In the present invention, however, because the band is designed to show a small stress at the time of vulcanisation, the decrease in the diameter is limited to a small percentage. Incidentally, if a steel cord is used in the belt, as the rigidity is higher than the non-metallic cords, the diameter is hardly decreased, but the tyre weight increases. If the change in the tyre diameter is large, the tensile stress by the application of inner pressure in actual service conditions causes a stress concentration, and as a result, cracks in the tread groove and cuts in the tread portion are caused by repeated deformation and injuries by objects. The present invention therefore, provides a pneumatic radial tyre in which, by decreasing the change in tyre diameter, durability is improved and total tyre performance is greatly improved. An embodiment of the present invention will now be described in conjunction with the drawings. Fig.1 is a cross sectional view of an embodiment of the present invention; Figs.2 and 3 are graphs showing the stress-elongation curves of the cords used in test tyres; Fig.4 is an exterior view of a hybrid cord No.2; Fig.5 is an exterior view of a hybrid cord No.6; Fig.6 is a perspective view of a narrow strip used to form the band belt ply; and Fig.7 is a cross sectional view explaining a method of winding the narrow strip. In Fig.1, a pneumatic radial tyre 1 comprises a tread portion 2, a sidewall portion 3 extending radially inwardly from each edge of the tread portion 2, a bead portion 4 located at the radially inner end of each of the sidewall portions 3, a bead core 5 disposed in each of the bead portions 4, a toroidal carcass 6 of which both edges are turned up around the bead cores 5 in the bead portions 4, a breaker belt 7 disposed radially outside the carcass 6 in the tread portion 2, and a band belt 8 disposed radially outside the breaker belt 7. The carcass 6 comprises at least one ply of cords arranged radially at an angle of form 60 to 90 degrees with respect to the tyre equator C. The breaker belt 7 in this embodiment comprises two plies 7A and 7B of aramid fibre cords, wherein the cords in each of the plies 7A and 7B are laid parallel with each other but crosswise to the cords in the next ply. The band belt 8 in this embodiment comprises a pair of edge bands 10 each covering each edge portion of the breaker belt 7 and a main band 9 disposed radially outside thereof covering the substantially full width of the breaker belt 7 including the edge bands. Each of the main band 9 and edge bands 10 is a JLB ply 8A made of at least one hybrid cord wound spirally and continuously in the circumferential direction of the tyre at an angle of from 0 to 3 degrees with respect to the tyre equator. In this embodiment, the JLB ply 8A is formed by spirally winding at least one long narrow strip 20 around the outer surface of the breaker belt 7. In the narrow strip 20, as shown in Fig.6, hybrid cords 23 are coated with a topping rubber 24 in a form of a ribbon. As shown in Fig.4 and Fig.5, the hybrid cord 23 comprises at least one high elastic modulus thread 21 and at least one low elastic modulus thread 22 which are twisted together. The high elastic modulus thread 21 is made of high elastic modulus fibre, and the low elastic modulus thread 22 is made of low elastic modulus fibre. The high elastic modulus fibre has an elastic modulus of not less than 3000 kgf/sq.mm, preferably not less than 5000 kgf/sq.mm. Preferably, aramid fibre is used, but polyvinyl alcohol fibre having a strength of not less than 15g/d, carbon fibre, glass fibre and the like may be used. If the elastic modulus of the high elastic modulus fibre is lower than 3000 kgf/sq.mm, a sufficient hoop effect can not be obtained, and the prevention of lifting during high speed running becomes insufficient. The low elastic modulus fibre has an elastic modulus of not more than 2000 kgf/sq.mm, preferably not more than 1000 kgf/sq.mm. Preferably, aliphatic polyamide fibre, i.e. nylon fibre is used. To form a high elastic modulus thread 21, one or more aramid fibres are first-twisted together in a certain direction. Similarly, to form a low elastic modulus thread 22, one or more nylon fibres are first-twisted together in the same direction as the aramid fibres. Then, those threads 21 and 22 are final-twisted together in the reverse direction to the first-twist so as to form a hybrid cord 23. Therefore, in such hybrid cord, the first-twisted threads are twisted back by the turns corresponding to the final-twist. When such a hybrid cord 23 is loaded, in the initial stage of the elongation, the cord is easily elongated with a very small stress, since the spiral twist of the hybrid cord 23 is first twisted back. However, when the elongation reaches to a specific value, a larger load is required to elongate it since the elongation is mainly resisted by the elastic modulus of each fibre. Table 1 shows the specifications of test tyres. The test tyres had a tyre size of 205/60R14 and the same tyre construction shown in Fig.1 except for the cords used in the JLB belt. The stress-elongation curves of the cords used in the JLB belt are shown in Fig.2 and Fig.3, and the specifications thereof are also shown in Table 1. In Table 1, the "Tyre diameter change" was the difference between the diameter at an inner pressure of 0.1 kgf/sq.cm and that at 2.0 kgf/sq.cm. The "Cut open width" was the maximum width of the opening of a cut, when the bottom of a tread main groove was cut with a knife by a length of 8 mm along the groove and a depth of 2 mm under a state that the test tyre was not inflated, and then the tyre was inflated to the regulated maximum inner pressure. The "Crack resistance" was found, using a drum tester, with the test tyre inflated to 1.9 kgf/sq.cm and loaded with 580 kgf (the maximum load to the approved maximum air pressure). The tyre was run for 30,000 kilometres at a speed of 80 kilometre/hour, blowing air including 80 pphm ozone against the tyre. The running distance until the bottom of the main grooves was cracked at at least five positions per one groove was measured. The larger the running distance, the higher the resistance. The "Steering stability" was an evaluation into five ranks made by a driver who drove a test car provided with the test tyres in a test course. The larger the value, the better the stability. The stress-elongation curve 1 of cord No.1 is in Table 1, which is a hybrid cord made of a high elastic modulus thread of 1000 denier aramid fibres first-twisted at 42 turns/10cm and a low elastic modulus thread of 1260 denier nylon-66 fibres first-twisted at 24.5 turns/10cm, which are final-twisted at 37.5 turns/10cm. Curve 2 is for cord No.2 which is a hybrid cord made of a high elastic modulus thread of 1000 denier aramid fibres first-twisted at 42 turns/10cm and a low elastic modulus thread of 840 denier nylon fibre first-twisted at 30 turns/10cm, which are final-twisted at 42 turns/10cm. In the JLB belt, the cord count (E) for the hybrid cord was 50 /5cm. Therefore, in the test tyres 1 and 2, the product (F1xE) of the stress (F1) of the hybrid cord at 2% elongation multiplied by the cord count (E) was set at a rather high value such as 120 (tyre 1) and 85 (tyre 2). As the decrease in the tyre diameter was large, tread groove cracks were found at more than five positions per one tread groove after running for 25,000 kilometres. The stress-elongation curve 3 is for the hybrid cord 3 in Table 1, in which the thickness of the low elastic modulus thread was 420 denier, the first-twist was 42.4 turns/10cm, and the final-twist was 47.8 turns/10cm. Accordingly, the apparent difference in the twist number was 5.8 turns/10cm in the direction of the final-twist of the high elastic modulus thread and 5.4 turns/10cm in the direction of the final-twist of the low elastic modulus thread. In this tyre 3, the product F1xE of the stress F1 at 2% elongation and the cord count E was 55. The tread groove cracks found after 30,000 kilometre running were at less than five positions. The stress-elongation curve 4 is for hybrid cord 4, in which the first-twist of the low elastic modulus thread was 20 turns/10cm which was smaller than 1/2 of the final-twist of 47.8 turns/10cm. In this tyre 4, the product F1xE of the stress F1 at 2% elongation and the cord count E was 40, and the product F2xE of the stress F2 at 6% elongation and the cord count E was 295. No tread groove crack was found even after running for 30,000 kilometres. The stress-elongation curve 6 is for hybrid cord 6, in which the high elastic modulus thread was 1000 denier, and the low elastic modulus thread was 840 denier. In this tyre 6, the product F1xE and F2xE were 60 and 280, respectively. After running for 30,000 kilometres, only slight cracks were found. The stress-elongation curve 7 is for hybrid cord 7, in which the low elastic modulus thread was 420 denier which was smaller than 1/2 of the high elastic modulus thread. In this tyre 7, no crack occurred. Curve 8 is for hybrid cord 8 in Table 8. In the tyre 8, steel cords were used in the breaker belt instead of aramid cords. The tyre diameter change was therefore small, and no crack occurred, but a tyre weight reduction was not achieved. The stress-elongation curve 9 is for cord 9 which was made of two twisted 1260 denier low elastic modulus threads. In this tyre 9, as the JLB belt was made of such low elastic modulus cords, the diameter change was large, but as the belt rigidity was low, no crack occurred. However, the tyre lacked hoop effect to resist during high speed running and steering stability. As explained above, in the tyre manufacturing process, it becomes possible to press the raw tyre onto the impression of the mould by increasing the inner pressure of the tyre. In the finished tyre, as a result of the high elastic modulus thread in the band belt, the tyre expansion in high speed running is decreased, and the tyre diameter change in the tyre manufacturing process is also decreased. Accordingly, the occurrence of the tread groove bottom cracks after high speed running can be prevented. Therefore, it is possible to provide a light-weight tyre having an excellent high speed durability. Further, as the JLB belt in the present invention is wound spirally, there is no splice, and the tyre is provided with uniformity in the circumferential direction. Therefore, vibrations during running become decreased.

Claims: A pneumatic radial tyre comprising a toroidal carcass ply (6), a breaker belt (7) disposed radially outside the carcass ply (6), and a band belt (8) disposed radially outside the breaker belt (7), whereby the breaker belt (7) comprising at least one ply (7A,7B) of parallel non-metallic cords, the band belt (8) comprising a ply of at least one hybrid cord (23) wound spirally and continuously in the circumferential direction of the tyre at an angle of 0 to 3 degrees with respect to the tyre equator, the hybrid cord (23) comprising a low elastic modulus thread (22) and a high elastic modulus thread (21), characterized in that the low elastic modulus thread (22) and the high elastic modulus thread (21) are final-twisted together, the low elastic modulus thread (22) comprising at least one low elastic modulus fibre first-twisted and having an elastic modulus of not more than 2000 kgf/sq.mm, the high elastic modulus thread (21) comprising at least one high elastic modulus fibre first-twisted and having an elastic modulus of not less than 3000 kgf/sq.mm, in the band belt ply (8), the cord count (E) of the hybrid cord per 5cm width, the stress (F1) in kgf of the hybrid cord at 2% elongation, and the stress (F2) in kgf of the hybrid cord at 6% elongation satisfying the following relationships: FI x E < 60; and F2 x E > 150. A pneumatic radial tyre according to claim 1, characterised in that the thickness (d1) in denier of the low elastic modulus thread (22) is not more than 1/2 of the thickness (d2) in denier of the high elastic modulus thread (21). A pneumatic radial tyre according to claim 1, characterised in that the first-twist (t1) of the low elastic modulus thread (22) is not more than 1/2 of the final-twist (t0) of the hybrid cord (23). A pneumatic radial tyre according to claim 1, 2, or 3, characterised in that the low elastic modulus fibre has an elastic modulus of not more than 1000 kgf/sq.mm, and the high elastic modulus fibre has an elastic modulus of not less than 5000 kgf/sq.mm. A pneumatic radial tyre according to claim 1, 2, 3 or 4, characterised in that the product ExF1 of the cord count E and the stress F1 (kgf) at 2% elongation of the hybrid cord is not more than 45, and the product ExF2 of the cord count E and the stress F2 (kgf) at 6% elongation of the hybrid cord is not less than 200. A pneumatic radial tyre according to claim 2, 3 or 4, characterised in that the hybrid cord consists of one low elastic modulus thread and one high elastic modulus thread which are final-twisted together. A pneumatic radial tyre according to any of claims 1, 2, 3 or 4, characterised in that the low elastic modulus fibre is made of nylon fibre, and the high elastic modulus fibre is made of aromatic polyamide fibre.

Inventors: MIYAZAKI SHINICHI, NAKAYASU RITSUO, MIYAZAKI, SHINICHI, NAKAYASU, RITSUO
Applicants: SUMITOMO RUBBER IND, SUMITOMO RUBBER INDUSTRIES LTD.
Application year: 1994
Main classification: B60C9
IPC codes: B60C9